Number of FRBs needed to detect a given σ_RM.IGM at two red-shifts (ɀ = 0.5 and ɀ = 2). We consider the IGMF possible to detect if the KS test returns a p-value below 0.05 for 95% of Monte Carlo tests (for details, see Sect. 4.1.2). The data points show the FRB number needed for significant detection for a given σ_RM.IGM, and the solid line is the reciprocal exponential fit. We also show the number of FRBs needed considering an RM error of 5 and 50 rad m⁻² as empty circles and squares, respectively. We indicate the observations of O’Sullivan et al. (2019) and Carretti et al. (2022).